[css-pseudo-4] Mark CSSPseudoElement interface under construction per w3c#4619 (comment)

Author: fantasai

css-pseudo-4/Overview.bs

@@ -1183,6 +1183,10 @@ Additions to the CSS Object Model</h2>
     };
   </pre>
 
+  ISSUE: This interface is under design development,
+  and this draft is looking for feedback more than implementation.
+  The CSSWG would particularly appreciate hearing about use cases and problems.
+
   The <dfn attribute for=CSSPseudoElement>type</dfn> attribute
   is a string representing the type of the pseudo-element.
   This can be one of the following values: